1. Field of the Invention
The present invention relates to a method for producing a silicon single crystal having a high uniformity of oxygen concentration in-plane distribution of single crystal at a high growth rate, and a silicon single crystal.
2. Description of the Related Art
Most of currently produced electronic devices such as processing devices and memories are manufactured on a wafer produced from a silicon single crystal pulled by the Czochralski method (CZ method). The CZ method is a method for growing a silicon single crystal by pulling it from a silicon melt fused in a crucible placed in a heater.
The crystal growth rate in the Czochralski method is determined by heat balance of growing crystal (see FIG. 1). The heat entering into the crystal consists of heat, Hin, transferred from the silicon melt to the crystal and solidification latent heat, Hsol, generated upon phase change of the melt from liquid to solid. As for the heat balance in the vicinity of crystal growing portion, the heat, Hout, emitted from the crystal is considered to be equal to the sum of Hin+Hsol. Because Hout increases in proportion to the crystal surface area, it depends on the crystal radius r [mm], and is proportional to .pi..multidot.r. Because the magnitude of Hin depends on the cross sectional area of the crystal, it is proportional to .pi..multidot.r.sup.2. Further, because the magnitude of the solidification latent heat Hsol depends on the volume of crystal growing per unit time, it is a function of .pi..multidot.r.sup.2 .multidot.V where V denotes crystal growth rate [mm/min]. Accordingly the above-mentioned equation is represented as the following equation: .alpha..multidot..pi..multidot.r=.beta..multidot..pi..multidot.r.sup.2 +.gamma..multidot..pi..multidot.r.sup.2 .multidot.V. This equation is transformed into an equation: EQU V=a/r+b (1).
In the equation (1), it has empirically been found that an average growth rate Vave applicable to industrial production of crystals can be obtained with a of 115 or less and b of 0. That is, it should be around 2.26 mm/min or less for a crystal having a diameter of 4 inches, 1.51 mm/min or less for a crystal having a diameter of 6 inches, 1.13 mm/min or less for a crystal having a diameter of 8 inches, and 0.75 mm/min or less for a crystal having a diameter of 12 inches.
As examples of prior art utilizing a relatively high pulling rate, Japanese Patent Application Nos. Sho 54-41161/1979 and Sho 59-187082/1984 can be mentioned. The pulling rate was 2-3 mm/min for a diameter of 80 mm [2.88 mm/min in the formula (1)] in the former application, and 2.3 mm/min for a diameter of 4 inches in the latter application. These techniques mainly rely on use of larger Hout mentioned above, which is obtained by increasing the temperature gradient along the axis direction (dT/dz)c in the vicinity of growing interface of the crystals. Theoretically, if this temperature gradient could limitlessly be made larger, the maximal value Vmax of the average growth rate could also be limitlessly made larger. However, actual use of steep temperature gradient may cause solidification of silicon melt, which impedes the use of higher growth rate. Therefore, such average growth rates as mentioned above are considered as the limit. That is, the actual limit of the average growth rate would be around 2.26 mm/min or less for a crystal having a diameter of 4 inches, 1.51 mm/min or less for a crystal having a diameter of 6 inches, 1.13 mm/min or less for a crystal having a diameter of 8 inches, and 0.75 mm/min or less for a crystal having a diameter of 12 inches.
Of course, if productivity and production yield are ignored, or quality is ignored on an experimental basis, it is possible to temporarily exceed the values mentioned above. For example, if an extremely short crystal is grown, the average crystal growth rate may exceed the rates mentioned above. Further, it is also possible to exceed the rates mentioned above by ignoring ununiformity of in-plane (radial) distribution of oxygen concentration, and using a lower rotation rate.
However, if a higher crystal growth rate is used in such methods, use of such constrained operation conditions may prevent single crystals from growing, and cause problems concerning quality, that may lead to undue reduction of productivity and production yield, generation of projections on crystal surfaces, failing in obtaining a cylindrical single crystal ingot to be processed into a wafer required to have a given diameter due to deformation of the crystals themselves, or the like. Such deformation may be suppressed to some extent by using a low angular rate around the crystal with a low rotation rate of the crystal. However, in such a case, in-plane distribution of oxygen concentration may disadvantageously be degraded to an undue extent to reach such a level that cannot be used for an industrial product. Eventually, it has not been able to produce silicon single crystals which could be used as industrial products at a growth rate higher than those rates mentioned above.